1. Field of the Invention
The present invention relates to checking the composition of non-metallic materials and, more particularly, to apparatus for measuring the magnetic content of non-metallic samples. The invention can be used in manufacture of abrasive materials, construction materials, electric-grade and radioceramics, and in production of foodstuffs such as flour and sugar.
2. Description of the Prior Art
Known in the prior art is an apparatus for determining the amount of magnetic material in a sample (U.S. Pat. No. 3,808,524) comprising a measuring frequency oscillator with the test sample placed within the coil, a reference frequency oscillator, a mixer having its inputs connected to the outputs of the oscillators and its ouput connected, through a low-pass filter, to the input of a frequency-to-voltage converter whose output is connected to an analog or digital voltmeter. The magnetic content of the sample is determined by the frequency difference between the measuring frequency oscillator and the reference frequency oscillator, after the test sample has been placed inside the coil of the measuring frequency oscillator.
This apparatus has a low sensitivity, preventing the check to be performed on test samples with a magnetic content of below 0.1%. The low sensitivity of the apparatus is due to the fact that the placement of a low magnetic content sample within the measuring frequency oscillator coil may fail to result in a frequency change of the measuring frequency oscillator because of the spurious coupling tending to exist between this oscillator and the reference frequency oscillator. In addition, the sensitivity of the apparatus is limited by the available frequency stability of the two oscillators.
An apparatus is known for measuring the magnetic content of non-magnetic samples, which comprises a pair of identical self-excited sine wave oscillators, namely, the operating and reference frequency oscillators, coupled by a mutually synchronizing circuit, and a dual-channel phase meter connected to the oscillator outputs (B. A. Glagovsky et al. "Kontrolno-izmeritelnye pribory i osnovy avtomatizatsii proizvodstva abrazivnykh instrumentov", Mashinostroenie, Leningrad, 1980, p. 126-127). The test sample is placed in the inductance coil of the resonant circuit of the operating oscillator, leading to a change in the tuning frequency of the resonant circuit. Since both the operating and the reference frequency oscillators are mutually synchronized, the oscillation frequency of the operating oscillator is not changed. Instead, a phase shift is generated between the oscillator waves which is measured by a phase meter. The value .phi. of the phase shift is given by: ##EQU1## wherein E.sub.m is the amplitude of oscillations in the resonant circuit of the operating oscillator,
E is the amplitude of synchronizing oscillations applied to the resonant circuit of the operating oscillator from the reference frequency oscillator through the mutualy synchronizing circuit, PA1 K is the proportionality coefficient dependent on the properties of the material to be tested and on the inductance coil parameters in the resonant circiut of the operating oscillator, and PA1 P is the magnetic content of the test sample.
The E.sub.m /E ratio, which is the coupling coefficient between the resonant circuits of the oscillators, depends on the resistance of the mutual synchronization loop. In this apparatus, the mutual synchronization loop is fixed, i.e. it has a constant coupling coefficient.
The coupling coefficient value is chosen so that the phase shift between the oscillations of the operating and reference frequency oscillators, for a specified magnetic content measurement range, will not exceed 90.degree.. If the phase shift between the oscillator waves is in excess of 90.degree., the mutual synchronization mode is disturbed.
The above expression shows that the phase angle between the oscillations of the two oscillators is a nonlinear function of the magnetic content P of the test sample, resulting in a measurement error. This error signal is comparatively insignificant on the initial portion of the curve plotted using the above expression, i.e. with small amounts of magnetic materials, but an abrupt rise occurs as the amount is increased. In actual devices of the type described, the upper limit of the measurement range with an acceptable error is only 0.1%, whereas the magnetic inclusion content of, say, abrasive materials may be as large as 3%. Thus the accuracy of the apparatus as described above is reduced as the value to be measured increases.